1. Field of the Invention
This invention relates to guided-mode resonance filters for producing efficient bandpass reflection and transmission with low sidebands.
2. Brief Description of the Relevant Art
Methods of dispersing wave energy are known in the art. In particular, spatially periodic elements, such as gratings, have been used to diffract wave energy, e.g., such as light incident on the element. Diffraction gratings can be used to influence the amplitude, phase, direction, polarization, spectral composition, and energy distribution of the electromagnetic wave. Examples of classical applications of gratings include deflection, coupling, filtering, multi-plexing, guiding, and modulation of optical beams in areas such as holography, integrated optics, optical communications, acousto-optics, nonlinear optics, and spectroscopy.
In general, the efficiency of a grating varies smoothly from one wavelength to another. However, there can be localized troughs or ridges in the efficiency curve and these are observed as rapid variations in efficiency with a small change in either wavelength or angular incidence. These troughs or ridges are sometimes called "anomalies." From the point of view of a spectroscopist, anomalies are a nuisance because they introduce various peaks and troughs into the observed spectrum. It is, therefore, very important that the positions and shapes of the anomalies be accurately predicted as well as the conditions under which they appear. However, as the present invention indicates, these "anomalies" may be employed to carry out some very useful purposes.
Guided-mode resonance effects in waveguide gratings generate sharp variations in the intensity of the observable propagating waves. This resonance results from evanescent diffracted waves that are parametrically near to a corresponding leaky mode of the waveguide grating. Because the propagating and evanescent diffracted waves of gratings are both coupled to the adjacent orders, a resonance in an evanescent wave can cause a redistribution of the energy in propagating waves. For high-efficiency resonance effects, the grating filters can be designed to admit only zero-order forward- and backward-propagating waves with primary contributions from the +1 and/or -1 order evanescent-wave resonances. At resonance, the diffraction efficiency of the forward-propagating wave approaches zero, and that of the backward wave tends to unity. Features of this guided-mode resonance effect, such as high-energy efficiency and narrow linewidth, may lead to applications in laser filtering technology, integrated optics, and photonics.
In 1965, Hessel and Oliner presented a mathematical model that analyzed reflection anomalies occurring at both the resonance and the Rayleigh wavelengths for a given equivalent surface reactance. [A. Hessel and A. A. Oliner, "A New Theory of Woods Anomalies" Appl. Opt., vol. 4, pp. 1275-1298, 1965.] Since then, others have studied grating anomalies and resonance phenomena on surface-relief gratings and corrugated dielectric waveguides. Many potential applications based on the narrow-line reflection filter behavior of the fundamental, planar waveguide grating structure have been described. [S. S. Wang and R. Magnusson, "Theory and applications of guided-mode resonance filters," Applied Optics, Vol. 32, pp. 2606-2613, 1993; S. S. Wang and R. Magnusson, "Multilayer waveguide-grating filters," Applied Optics, Vol. 34, pp. 2414-2420, 1995.]
Some of the above problems were addressed by Magnusson al., U.S. Pat. No. 5,216,680 through the use of an optical guided-mode resonance filter. This filter provided predictable anomalous effects such that an accurate passband having known diffraction efficiency could be designed. However, this patent did not address the elimination of the filter sidebands. Furthermore, this patent did not address transmissive filters.